Definitions include: Expression of disappointment, similar to 'bummer', though more forceful. 'Bummer' tends to be used when something unforeseen and unfortunate occurs. 'Rats', on the other hand tends to be used when you have been making a concerted effort to do something and it suddenly fails. You only use 'rats' about your own misfortune. You can use 'bummer' sympathetically also about someone else's misfortune.